A conventionally known latching relay drive circuit is a one in which a capacitor is disposed in series to an operation coil disposed in a single winding latching relay (Patent Documents 1 and 2).
(Configuration of Conventional Latching Relay Drive Circuit)
FIG. 9 is a circuit diagram illustrating a configuration of a conventional latching relay drive circuit disclosed in Patent Document 1. The latching relay drive circuit includes a power supply 51, a current control resistor 52, a power switch 53, a load 55, and a hybrid relay 54 for open-close controlling the load 55. This hybrid relay 54 is configured in such a manner that a series circuit including an operation coil 57 of a latching relay and a capacitor 58 is connected to output terminals of a Schmitt circuit 56, and a transistor 59 for recovering this operation coil 57 is connected in parallel. The hybrid relay 54 is disposed with a base resistor 60 for the transistor 59 and a diode 61 for off-operating the transistor 59. A relay contact 62 for the latching relay is disposed between the power switch 53 and the load 55.
(Operation of Conventional Latching Relay Drive Circuit)
First, when the power switch 53 is closed, power is supplied from the power supply 51, via the Schmitt circuit 56, to the operation coil 57, and the power remains until the capacitor 58 is fully charged. By the power to this operation coil 57, its relay contact 62 turns on, thus the power is supplied from the power supply 51, via the relay contact 62, to the load 55. When the power is supplied to the above-described operation coil 57, a current flows in a forward direction to the diode 61.
As a result, no potential difference occurs between a base and an emitter of the transistor 59, thus this transistor 59 does not on-operate, but the power is supplied to the operation coil 57.
Next, when the power supply switch 53 is open, a charging voltage in the capacitor 58 is applied in a backward direction to the diode 61. When this reverse voltage is applied between the base and the emitter of the transistor 59, this transistor 59 on-operates to allow a charging current to instantaneously flow in a backward direction from the capacitor 58 to the latching relay 57. Accordingly, the relay contact 62 turns off to shut off the power to the load 55 at a high speed.
(Configuration of Another Conventional Latching Relay Drive Circuit)
FIG. 10 is a circuit diagram illustrating a configuration of another conventional latching relay drive circuit, disclosed in Patent Document 2. This latching relay drive circuit includes an alternating current power supply AC. Both ends of the alternating current power supply AC are connected with a surge absorber ZN via a switch SW. Both ends of the surge absorber ZN are connected with a full-wave rectifying circuit DB including a diode bridge, via a resistor Rs for protecting from a surge current.
Between output terminals of this full-wave rectifying circuit DB, collectors and emitters of transistors Tr71 and Tr72, a diode D71, a capacitor C71, and an operation coil Ly of a single winding latching relay are sequentially connected in series so as to configure a constant voltage circuit. A resistor R71 is connected between the collector and a base of the transistor Tr71, and a resistor R72 is connected between the base of the transistor Tr71 and a base of the transistor Tr72. Between the base of the transistor Tr72 and a negative pole output end of the full-wave rectifying circuit DB, a Zener diode ZD is connected.
A smoothing capacitor C72 configuring a delay circuit, and a series circuit including voltage-dividing resistors R73 and R74 are connected in parallel between the emitter of the transistor Tr72 and the negative pole output end of the full-wave rectifying circuit DB. A coupling point between the resistor R73 and the resistor R74 is connected to a base of a transistor Tr73 that connects its emitter to the negative pole output end of the full-wave rectifying circuit DB.
Between an end of the capacitor C72 and a collector of the transistor Tr73, a series circuit including a diode D72, a resistor R75, and a base and an emitter of a transistor Tr4, and another series circuit including a diode D73, a resistor R76, and a collector and an emitter of a transistor Tr75 are connected.
A cathode of the diode D73 is connected to a base of a transistor Tr76. An emitter of the transistor Tr76 is connected to a cathode of the diode D71. A collector of the transistor Tr76 is connected to both of a base of the transistor Tr75 and a collector of a transistor Tr74. Between the emitter and the collector of the transistor Tr76, a resistor R77 is connected to provide a higher resistance.
The transistor Tr74 configures a switching circuit to control a thyristor structure including the transistors Tr75 and Tr76.
(Operation of Another Conventional Latching Relay Drive Circuit)
First, when the switch SW is closed, the full-wave rectifying circuit DB rectifies an alternating-current voltage. The rectified voltage is then smoothed by the capacitor C72, via the constant voltage circuit including the transistors Tr71 and Tr72, the resistors R71 and R72, and the Zener diode ZD. When this direct current voltage is divided by the resistors R73 and R74, and the voltage between both ends of the resistor R74 reaches a value between 0.6 and 0.7 V, the transistor Tr73 comes on. And then, a charging current of the capacitor C72 flows from a point “a” shown in FIG. 10, via the diode D71, the capacitor C71, and the operation coil Ly, toward the transistor Tr73, so that the latching relay is set, i.e. is on-operated.
Next, when the switch SW is open, an electric charge in the capacitor C72 discharges via the resistors R73 and R74. Meanwhile the voltage between both the ends of the resistor R74 gradually drops, and then the transistor Tr73 goes off. As the transistor Tr73 goes off, the transistor Tr74 configuring the switching circuit also goes off, thus a potential at the collector of the transistor Tr74 quickly rises. That is, a positive pulse is applied to a gate (a point “b” shown in FIG. 10) of the thyristor structure including the transistors Tr75 and Tr76, and the transistors Tr75 and Tr76 quickly come on to discharge an electric charge from the capacitor C71 via the transistors Tr75 and Tr76.
As a result, a discharge current (reset current) flows from the capacitor C71, via the transistors Tr76 and Tr75, toward the operation coil Ly so that the latching relay is reset, i.e. is off-operated.